A local expansion technique for the reconstruction of the plasma boundary is presented. The method is particularly accurate in identifying the separatrix in X-point configurations. It is applied to JET discharges and the results compared with those of a full equilibrium code and with other independent diagnostics. It is found that the majority of the H-mode discharges at JET have been achieved in marginal limiter configurations. The method is sufficiently reliable and fast for real time shape control.